This issue has been going on for a while and never got fixed. Using KDE Dragonite version, the bottom shortcut bar sometimes display double, as shown in this picture.
I'm using HDMI output display on a dual-graphics laptop. Sometimes after opening or closing some apps, the 2nd bar appears.
I often use META+Tab to switch between 2 activities, and the 2 activities have different icons available, which causes a weird non-smooth animation to hide/show the icons. That's another issue I'd love to see improved, but may or may not be related.
When this but occurs, the only solution is to reboot the computer.
This bug also causes weird behaviors of how the 2 shortcut bars interact with each other, like the gray bar below it flickering when hovering icons, and either 1 or 2 bars being visible over applications depending on actions.